The description of prior art will be primarily based on the references listed below:
List of References
    1. D. L. Foster, “Separator for lithium batteries and lithium batteries including the separator,” U.S. Pat. No. 4,812,375, Mar. 14, 1989.    2. D. H. Shen, et al. “Dendrite preventing separator for secondary lithium batteries,” U.S. Pat. No. 5,427,872, Jun. 27, 1995.    3. F. Goebel, et al., “Getter Electrodes and Improved Electrochemical Cell Containing the Same,” U.S. Pat. No. 5,006,428 (Apr. 9, 1991).    4. D. Fauteux, et al., “Secondary Electrolytic Cell and Electrolytic Process,” U.S. Pat. No. 5,434,021 (Jul. 18, 1995).    5. M. Alamgir, et al. “Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat. No. 5,536,599, Jul. 16, 1996.    6. S. Kawakami, et al., “Secondary batteries,” U.S. Pat. No. 5,824,434, Oct. 20, 1998.    7. S. Kawakami, et al., “High energy density secondary battery for repeated use,” U.S. Pat. No. 6,395,423, May 28, 2000.    8. S. Kawakami, et al., “Rechargeable batteries,” U.S. Pat. No. 6,596,432, Jul. 22, 2003.    9. Z. Zhang, “Separator for a high energy rechargeable lithium battery,” U.S. Pat. No. 6,432,586, Aug. 13, 2002.    10. T. A. Skotheim, “Stabilized Anode for Lithium-Polymer Battery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No. 5,961,672 (Oct. 5, 1999).    11. Q. Ying, et al., “Protective Coating for Separators for Electrochemical Cells,” U.S. Pat. No. 6,194,098 (Feb. 27, 2001).    12. T. A. Skotheim, et al. “Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May 11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No. 6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007).    13. E. M. Shembel, et al., “Non-aqueous Electrolytes Based on Organosilicon Ammonium Derivatives for High-Energy Power Sources,” U.S. Pat. No. 6,803,152 (Oct. 12, 2004).    14. H. Kim, et al., “Non-aqueous Electrolyte and Lithium Battery Using the Same,” U.S. Pat. No. 7,244,531 (Jul. 17, 2007).    15. Y. S, Nimon, et al., “Dioxolane as a Protector for Lithium Electrodes,” U.S. Pat. No. 6,225,002 (May 1, 2001).    16. Y. S, Nimon, et al., “Methods and Reagents for Enhancing the Cycling Efficiency of Lithium Polymer Batteries,” U.S. Pat. No. 6,017,651 (Jan. 25, 2000); U.S. Pat. No. 6,165,644 (Dec. 26, 2000); and U.S. Pat. No. 6,537,701 (Mar. 25, 2003).    17. S. J. Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat. No. 6,025,094 (Feb. 15, 2000).    18. S. J. Visco, et al., “Protected Active Metal Electrode and Battery Cell Structures with Non-aqueous Interlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S. Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct. 16, 2007).    19. A. J. Bhattacharyya and J. Maier, “Non-aqueous Electrolyte for Use in a Battery,” U.S. patent application Ser. No. 10/919,959 (Aug. 6, 2004).    20. D. J. Burton, et al, “Method of Depositing Silicon on Carbon Materials and Forming an Anode for Use in Lithium Ion Batteries,” US Pub No. 2008/0261116 (Oct. 23, 2008).    21. D. W. Firsich, “Silicon-Modified Nanofiber Paper As an Anode Material for a Lithium Ion Battery,” US Patent Publication 2009/0068553 (Mar. 23, 2009).    22. Aruna Zhamu and Bor Z. Jang, “Hybrid Nano Filament Anode Compositions for Lithium Ion Batteries,” U.S. patent application Ser. No. 12/006,209 (Jan. 2, 2008).    23. Aruna Zhamu and Bor Z. Jang, “Hybrid Nano Filament Cathode Compositions for Lithium Ion and Lithium Metal Batteries,” U.S. patent application Ser. No. 12/009,259 (Jan. 18, 2008).    24. Aruna Zhamu and Bor Z. Jang, “Nano-structured Anode Compositions for Lithium Metal and Lithium-Air Secondary Batteries,” U.S. patent application Ser. No. 12/589,999 (Nov. 2, 2009).Lithium Metal Secondary Batteries:
Lithium-ion and lithium (Li) metal cells (including Li metal-air or, simply, Li-air cells) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium metal has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound (except Li4.4Si) as an anode active material. Hence, in general, Li metal (including Li-air) batteries have a significantly higher energy density and power density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having high specific capacities, such as TiS2, MoS2, MnO2, CoO2 and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. Specific cyclic stability and safety issues of lithium metal rechargeable batteries are primarily related to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway.
Many attempts have been made to address the dendrite-related issues, as summarized below:
Foster [Ref. 1] proposed a multilayer separator that included a porous membrane and an electro-active polymeric material contained within the separator materials. The polymer is capable of reacting with any lithium dendrite that might penetrate the separator, thus preventing the growth of dendrites from the anode to cathode that otherwise would cause internal shorting.
In a technically similar fashion, Shen, et al. [Ref. 2], used a non-reactive first porous separator (e.g., porous polypropylene) adjacent to the lithium anode and a second fluoro-polymer separator between the cathode and the first separator. The second separator (e.g., polytetrafluoro ethylene) is reactive with lithium. As the tip of a lithium dendrite comes into contact with the second separator, an exothermic reaction occurs locally between the lithium dendrite and the fluoro-polymer separator, resulting in the prevention of the dendrite propagation to the cathode.
Goebel, et al. [Ref. 3], proposed a “getter” electrode positioned between the anode and the cathode and was separated from the cathode and anode by fiberglass paper separators. The getter layer, composed of carbon or graphite material disposed on surfaces of these separators, serves as a low-capacity cathode that quickly discharges any Li dendrite that comes in contact with the getter layer.
Fauteux, et al. [Ref. 4], applied to a metal anode a protective surface layer (e.g., a mixture of polynuclear aromatic and polyethylene oxide) that enables transfer of metal ions from the metal anode to the electrolyte and back. The surface layer is also electronically conductive so that the ions will be uniformly attracted back onto the metal anode during electrodeposition.
Alamgir, et al. [Ref. 5], used ferrocenes to prevent chemical overcharge and dendrite formation in a solid polymer electrolyte-based rechargeable battery.
Kawakami, et al. [Ref. 6], observed that internal shorting could be prevented by using a multi-layered metal oxide film as a separator with small apertures through which lithium ions can pass and the growth of dendrites can be inhibited. Kawakami, et al. [Ref. 7], further suggested that the use of a first thin-film coating on the anode and a second thin film coating on the cathode, with both coatings permeable to lithium ions, could be effective in preventing dendrite formation. The first film can contain a large ring compound, an aromatic hydrocarbon, a fluoro-polymer, a glassy metal oxide, a cross-linked polymer, or a conductive powder dispersion. However, the dendrite-preventing mechanisms of these films were not clearly explained. Kawakami, et al. [Ref. 8], also found that some size mismatch between the anode and the cathode (with the anode being larger) seems to be effective in preventing dendrite formation.
Zhang [Ref. 9] disclosed a separator that is composed of a ceramic composite layer (to block dendrite growth) and a polymer micro-porous layer (to block ionic flow between the anode and cathode in the event of a thermal runaway).
Skotheim [Ref. 10] provided a Li metal anode that was stabilized against the dendrite formation by the use of a vacuum-evaporated thin film of a Li ion-conducting polymer interposed between the Li metal anode and the electrolyte. Ying, et al. [Ref. 11], proposed a separator that comprises a micro-porous pseudo-boehmite layer and a polymer-based protective coating layer. It was speculated that this separator had a small pore structure (10 μm or less) and sufficient mechanical strength to prevent the Li dendrite from contacting the cathode and causing internal shorting. Skotheim, et al. [Ref. 12], proposed a multilayer anode structure consisting of a Li metal-based first layer, a second layer of a temporary protective metal (e.g., Cu, Mg, and Al), and a third layer that is composed of at least one layer (typically 2 or more layers) of a single ion-conducting glass, such as lithium silicate and lithium phosphate, or polymer. It is clear that such an anode active material, consisting of at least 3 or 4 layers, is too complex and too costly to make and use.
Protective coatings for Li anodes, such as glassy surface layers of LiI— Li3PO4—P2S5, may be obtained from plasma assisted deposition [Ref. 17]. Complex, multi-layer protective coatings were also proposed by Visco, et al. [Ref. 18].
Organic additives that were used to stabilize the lithium anode active surface include (a) an organosilicon backbone with pyridinium groups bound to the backbone [Ref. 13], (b) halogenated organic metal salts [Ref. 14], and (c) dioxolane [Ref. 15]. Nimon, et al. [Ref. 16], developed methods and reagents for enhancing the cycling efficiency of lithium polymer batteries. The methods entailed forming a protective layer (e.g., LiAlCl4.3SO4 and Al2S3) on the lithium metal anode surface through a reaction of electrolyte species with lithium metal.
Despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. Clearly, an urgent need exists for a simpler, more cost-effective, and easier to implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries and other rechargeable batteries.
Lithium Ion Secondary Batteries:
Parallel to these efforts and prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to this replacement, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized.
The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g. In other graphitized carbon materials than pure graphite crystals, there exists a certain amount of graphite crystallites dispersed in or bonded by an amorphous or disordered carbon matrix phase. The amorphous phase typically can store lithium to a specific capacity level higher than 372 mAh/g, up to 700 mAh/g in some cases, although a specific capacity higher than 1,000 mAh/g has been sporadically reported. Hence, the magnitude of x in a carbonaceous material LixC6 varies with the proportion of graphite crystallites and can be manipulated by using different processing conditions. An amorphous carbon phase alone tends to exhibit a low electrical conductivity (high charge transfer resistance) and, hence, a high polarization or internal power loss. Conventional amorphous carbon-based anode materials also tend to give rise to a high irreversible capacity.
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential lithium ion anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a≦5 when the battery is fully charged) has been investigated as potential anode materials. This class of anode material has a higher theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, for the anodes composed of these materials, pulverization (fragmentation of the alloy particles) proceeds with the progress of the charging and discharging cycles due to expansion and contraction of the anode during the absorption and desorption of the lithium ions. The expansion and contraction also tend to result in reduction in or loss of particle-to-particle contacts or contacts between the anode and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, composites composed of small, electrochemically active particles supported by less active or non-active matrices have been proposed for use as an anode material. Examples of these active particles are Si, Sn, and SnO2. However, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction cycles, and some undesirable side effects.
Lithium Metal-Air Secondary Cells:
Metal-air batteries are unique in that they make use of oxygen from the atmosphere as the cathode reactant. A cathode active material is not required to be included in the cell because the oxygen consumed at the cathode is taken from the ambient. This feature allows metal-air cells to have extremely high energy densities. Among all metal elements, the metal with the highest operational voltage and greatest theoretical specific energy is lithium. However, there have been several major issues associated with the construction of lithium-air cells:
First, the utilization of an aqueous electrolyte has not been feasible due to corrosion of the lithium metal anode by water. This issue was addressed by Abraham and Jiang, U.S. Pat. No. 5,510,209, who demonstrated a cell with a non-aqueous polymer separator consisting of a film of polyacrylonitrile swollen with electrolyte solution of propylene carbonate/ethylene carbonate/LiPF6. This organic electrolyte membrane was sandwiched between a lithium metal foil anode and a carbon composite cathode to form the lithium-air cell. The utilization of the organic electrolyte allowed good performance of the cell in an oxygen or dry air atmosphere. The cells were reported to deliver a specific energy of 250-350 Wh/kg, based on the mass of the electrodes and electrolytes but not including the mass of the envelope package.
However, this lithium-air cell was plagued by a second issue: the capacity was limited by the formation of the Li2O discharge product which eventually blocked the pores of the carbon cathode, which was composed of graphite powder supported by a nickel screen. Furthermore, lithium-air secondary cells are also subject to the same dendrite-related issues as lithium metal secondary cells.
Highest Specific Capacity Anode Materials
Most significantly, lithium metal (including pure lithium, alloys of lithium with other metal elements, or lithium-containing compounds) still provides the highest anode specific capacity as compared to essentially all anode active materials (except pure silicon, but silicon has pulverization issues discussed above). Lithium metal would be an ideal anode material in a lithium metal or lithium-air secondary battery if dendrite related issues could be addressed.
It may be noted that in both lithium-ion and lithium metal secondary batteries, it is lithium ions that run back and force between the anode and the cathode. The amount of lithium pre-stored in the electrodes (anode and cathode) ultimately dictates the cell capacity and energy. In a Li-ion cell, the needed amount of lithium is normally fully stored in the cathode active material (e.g., lithiated cobalt oxide) when a cell is made due to the notion that cathode active materials, such as lithium transition metal oxide and lithium transition metal phosphate, are relatively stable in open air and can be more easily handled in a real manufacturing environment. Using pure Si as an example, the anode is free of lithium and the anode active material is 100% Si to begin with (before the first charging operation). The anode active material becomes Li4.4Si when the anode is fully charged (with lithium ions supplied from the pre-lithiated cathode active material). Such a conventional practice of storing lithium in the cathode has several drawbacks:
First, most of the cathode active materials (e.g., lithium cobalt oxide and lithium iron phosphate) have a very low specific capacity (typically in the range of 130-170 mAh/g) and, hence, a larger amount of cathode (than anode) materials has to be packed into a cell. It is desirable to have lithium-free cathode active material so that more cathode active material can be incorporated if lithium is stored at the anode side. Second, this practice also precludes the use of several high capacity cathode materials since these materials cannot be conveniently formed in a lithiated state.
In order to overcome these issues, in a slightly earlier application [Ref. 24], we incorporated surface-stabilized fine lithium particles as the anode active material mixed in a nano-structure of nano-filaments. As opposed to using surface-stabilized particles (which remain quite expensive), the instant application makes use of substantially pure lithium or lithium alloy that is in a foil form (preferred) or in a lithium rod form. Optionally, the lithium foil or rod can be surface-passivated or slightly alloyed or reacted with other elements to increase the air stability for easy handling of the anode during cell manufacturing. However, surface passivation is not a necessary requirement in the instant application. The conductive nano-filaments used in the anode of the instant application, to be discussed in the next section, are intended for serving as a substrate on which lithium will be deposited as a thin coating after the first charging operation and during subsequent re-charges. Although some of the nano-filaments, such as carbon nano-fibers, can be intercalated by lithium ions, this intercalation is not a primary function of these nano-filaments.
Hence, an object of the present invention was to provide a simple (not too complex), cost-effective, and easier-to-implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal or Li-air batteries.
Another object of the present invention was to provide a nano-structured composition for use as a Li metal anode that is resistant to dendrite formation and provides a Li metal cell that exhibits a long and stable cycling response.
Still another object of the present invention was to provide a lithium metal cell that exhibits a high specific capacity, high specific energy, good resistance to dendrite formation, and a long and stable cycle life.
A further object of the present invention was to provide a Li metal secondary cell wherein both the anode and the cathode comprise an integrated nano-structure of conductive nano-filaments with a lithium foil disposed at the anode as a primary source of lithium ions and a cathode active material bonded to or coated on surfaces of the nano-filaments.
Yet another object of the present invention was to provide a lithium-air cell that exhibits a high specific capacity, high specific energy, good resistance to dendrite formation, and a long and stable cycle life.
Yet another object of the present invention was to provide a lithium-air cell wherein the anode comprises a nano-structured composition (integrated structure of conductive nano-filaments) and a lithium foil, and the air cathode comprises an integrated structure of conductive nano-filaments. The integrated structure, being highly conductive, can also function as a current collector, obviating the need to have a separate current collector at either the anode or the cathode side. Such a configuration can significantly reduce the overhead weights of a cell, thereby affording an ultra-high specific capacity and specific energy.